The principal objectives of this research project are two-fold: 1) to determine host and viral factors that influence the genetically controlled resistance of inbred mouse strains to street rabies virus (SRV) and 2) to determine the mechanism(s) by which recombinant vaccines expressing different structural proteins of the rabies virus protect highly susceptible strains of mice against lethal infection. In a continuation of our cytotoxic T lymphocytes (CTL) studies, we have utilized monoclonal antibodies specific for CD4+ or CD8+ subsets of T cells to deplete the respective cell population in mice of the SJL/J and BALB/cByJ strains that are naturally resistant to SRV. Elimination of CD4+ T-helper cells abrogated the production of immunoglobulin G (IgG) neutralizing antibodies that occurs during rabies virus infection and reversed the resistant status of SJL/J and BALB/cByJ mice. In contrast, in vivo depletion of CD8+ cytotoxic T cells had no measurable effect of host resistance to SRV. Studies with recombinant vaccines expressing rabies virus structural proteins have shown that SRV susceptible mice vaccinated with raccoon poxvirus (RCN) recombinant viruses expressing the nucleoprotein (N) of the rabies virus are resistant to lethal challenge with SRV. Maximum survival was achieved following vaccination by tail scratch and footpad (FP) challenge. Mice immunized intraperitoneally were not protected. RCN-N- vaccinated mice inoculated in the FP with SRV were resistant to infection for at least 54 weeks postvaccination. Protection was also elicited by RCN recombinants expressing the rabies virus glycoproteins (RCN-G). Vaccination with RCN-G evoked rabies virus neutralizing antibody. Rabies virus neutralizing antibody was not detected in RCN-N-vaccinated mice prior to or following SRV infection. Sera from RCN-N-vaccinated mice which survived SRV infection did not contain antibody to SRV structural proteins G, M, or NS. The mechanism(s) of N-induced resistance correlates with the failure of peripherally inoculated SRV to enter the central nervous system (CNS). Support for this correlation with resistance was documented by the observations that SRV-inoculated RCN-N-vaccinated mice did not develop clinical signs of CNS rabies virus infection, infectious SRV was not detected in the spinal cord or brain following FP challenge, and 100% of RCN-N-vaccinated mice died following direct infection of the CNS with SRV via the intranasal or intracranial route. To date we have determined that the resistance of N-vaccinated mice does not appear to be due to interferon, NK cells, cytotoxic antibody, CTLs, or antibody-dependent-cell- mediated cytotoxicity. The search for the mechanism(s) of resistance continues. These results suggest that factors other than anti-G neutralizing antibody are important in resistance to rabies virus and that the N protein, because of its remarkable sequence conservation among N proteins of different rabies viruses, should be considered for incorporation with the G protein in recombinant vaccines. We are continuing to investigate the importance of CTLs in resolving rabies infections within the CNS, the mechanism(s) of immunity induced by recombinant vaccines expressing the rabies virus N, and the possibility that anti-rabies virus antibody localized within neurons and microglia affects the outcome of infection. GRANT-Z01AI00074 This project is focussed on the mechanisms of pathogenesis and immunity involved in Friend murine retrovirus-induced leukemia. In mice inoculated neonatally with Friend murine leukemia helper retrovirus two distinct pathogenic mechanisms have been identified: early hemolytic anemia with increased erythropoiesis and erythroleukemia with blocked erythropoiesis. Recent results using viral variants have shown that hemolytic anemia is altered in viral clones with a single amino acid difference in the envelope protein. Similarly lack of a second direct repeat enhancer sequence in the viral LTR decreased the anemia. These results should lead to a better understanding of the mechanisms by which retroviruses lyse infected cells in vivo. Studies on the role of the host immune system in protection of mice against Friend virus leukemia have focussed recently on the roles of different viral protein as vaccines. Purified viral envelope glycoprotein was found to be the best virion protein immunogen. In associated with a strong adjuvant this glycoprotein was capable of protecting both H-2a/b and H-2a/a mice. Protection of H-2a/a was surprising because these mice were not protected by vaccinia virus expressing Friend virus envelope protein. The results indicated that purified denatured envelope protein plus complete Freund's adjuvant induced both a T helper cell response and accelerated switching of anti viral antibodies from the IgM to IgG isotype.